Thermal Stratification and Its Impacts on Water Quality in Shahe Reservoir, Liyang, China

Abstract

Based on the one-year hourly water temperature profiles and the associated environmental drivers during the past eight years in Shahe Reservoir, Jiangsu Province, China from 2009 to 2016, the factors underlying the seasonal variation of thermal stratification and water quality response were investigated. It was shown that the thermal stratification was a typical subtropical one-cycle mixing model, lasting from May to September. The thermal stratification appeared and disappeared when the surface water temperature was 21 ℃ in the late spring and 19 ℃ in the middle of autumn. The difference between the water temperature at the epilimnion and hypolimnion increased with increasing solar radiation. When the air temperature was above 30 ℃, the stability of the thermal stratification increased. Heavy storms reduced the temperature of the surface water and weakened the temperature stratification of the column above a 5 m depth but had limited impact on the stratification of the hypolimnion deeper than 5 m. The thermal stratification greatly impacted the water quality of the lake. Hypoxia in the bottom water occurred by thermal stratification, leading to increased NH4+-N. The concentrations of dissolved oxygen, total phosphorus, and suspended solids in the hypolimnion increased after the disappearance of thermal stratification. Our results indicated that the thermal stratification was mainly controlled by solar radiation and the thermal stratification favored the growth of cyanobacteria and led to the release of nutrients from the sediment, threatening the water quality. Attention should be paid to thermal stratification to prevent algal blooms and related water quality deterioration.